The bacterial flagellum is a large, complex structure that is used by many bacteria as a motility organelle. It consists of three main substructures: the basal body, the hook and the filament. Most of the flagellar proteins are initially localized inside of the cell and translocated across the cell membrane by the flagellum-specific secretion apparatus that is evolutionarily and structurally related to the virulence type III secretion system (Non-Patent Literature 1 and 2). Protein export by the flagellar type III secretion system is highly regulated. The secretion system first exports rod/hook type proteins until the hook reaches an appropriate length. After that the secretion system switches substrate specificity from hook-type export to filament-type export (Non-Patent Literature 3 and 4). In Salmonella, the substrate-specificity switch is controlled by two proteins: FliK and FlhB (Non-Patent Literature 5-7).
FlhB is an essential membrane protein of the flagellar type III secretion system. It consists of two domains: a hydrophobic N-terminal part (FlhB™) that is predicted to have four transmembrane helices, and a C-terminal cytoplasmic domain (FlhBc) (Non-Patent Literature 8). The two domains are connected by a flexible linker. This linker is a highly conserved part of the FlhB protein and is essential for the type III secretion system (TTSS). Deletions or point mutations in the linker region completely abort or significantly reduce secretion (Non-Patent Literature 9 and 10). The wild-type cytoplasmic domain of Salmonella FlhB undergoes autocatalytic cleavage between amino-acid residues Asn269 and Pro270 within a highly conserved NPTH sequence (Non-Patent Literature 11). This auto-cleavage is essential for the switching process (Non-Patent Literature 9 and 12). Mutation of Asn269 to Ala prevents cleavage and locks the export apparatus in the hook-type specificity state.
FlhBC has been shown to interact with several soluble components of the TTSS: FliH, FliI, FliJ (Non-Patent Literature 13), the cytoplasmic part of membrane protein FlhA (Non-Patent Literature 14) and the hook-length control protein FliK (Non-Patent Literature 15 and 16). Interaction of FlhB with FliK is suggested to be important for the substrate-specificity switching process (Non-Patent Literature 17). Cells with a deleted fliK gene produce an abnormally long hook, termed a “polyhook”, without any filament attached (Non-Patent Literature 6).